Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is converted into organic molecules containing carbon and hydrogen. Those organic molecules containing only carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen known as oxygenates may be formed during the Fischer-Tropsch process. Hydrocarbons having carbons linked in a straight chain are known as aliphatic hydrocarbons that may include paraffins and/or olefins. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reactor.
Typically, the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Therefore, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the product molecular weight distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, as well as middle distillates. Hydrocarbon waxes may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons. Consequently, in the production of a Fischer-Tropsch product stream for processing to a fuel, it is desirable to maximize the production of high value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
Typically, in the Fischer-Tropsch synthesis, the product spectra can be described by likening the Fischer-Tropsch reaction to a polymerization reaction with a Shultz-Flory chain growth probability, called alpha value (α), that is independent of the number of carbon atoms in the lengthening molecule. The alpha value is typically interpreted as the ratio of the mole fraction of the Cn+1 product to the mole fraction of the Cn product. An alpha value of at least 0.72 is desirable for producing high carbon-length hydrocarbons, such as those of diesel fractions.
The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification).
Cobalt metal is particularly desirable in catalysts used in converting natural gas to heavy hydrocarbons suitable for the production of diesel fuel. Alternatively, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but has the disadvantage of a water-gas shift activity. Ruthenium has the advantage of high activity but is quite expensive.
Catalysts often further employ a promoter in conjunction with the principal catalytic metal. A promoter typically improves a measure of the performance of a catalyst, such as activity, stability, selectivity, reducibility, or regenerability.
Further, in addition to the catalytic metal, a Fischer-Tropsch catalyst often includes a support material. The support is typically a porous material that provides mechanical strength and a high surface area, in which the active metal and promoter(s) can be deposited. In a common method of loading a Fischer-Tropsch metal to a support, the support is impregnated with a solution containing a dissolved metal-containing compound. The metal may be impregnated in a single impregnation, drying and calcination step or in multiple steps. When a promoter is used, an impregnation solution may further contain a promoter-containing compound. After drying the support, the resulting catalyst precursor is calcined, typically by heating in an oxidizing atmosphere, to decompose the metal-containing compound to a metal oxide. When the catalytic metal is cobalt, the catalyst precursor is then typically reduced in hydrogen to convert the oxide compound to reduced “metallic” metal. When the catalyst includes a promoter, the reduction conditions may cause reduction of the promoter, or the promoter may remain as an oxide compound.
Catalyst supports for catalysts used in Fischer-Tropsch synthesis of hydrocarbons have typically been refractory oxides (e.g., silica, alumina, titania, zirconia or mixtures thereof). It has been asserted that the Fischer-Tropsch synthesis reaction is only weakly dependent on the chemical identity of the metal oxide support (see E. Iglesia et al. 1993, In: “Computer-Aided Design of Catalysts,” ed. E. R. Becker et al., p. 215, New York, Marcel Dekker, Inc.). Nevertheless, because it continues to be desirable to improve the performance of Fischer-Tropsch catalysts, other types of catalyst supports are being investigated.
In particular, various aluminum oxide compounds have been investigated. For example, gamma-alumina is an oxide compound of aluminum having, in its pure form, the empirical formula γ-Al2O3. Gamma-alumina distinguished from other polymorphic forms of alumina, such as alpha-alumina (α-Al2O3), by its structure, which may be detected for example by x-ray diffraction (see for example Zhou & Snyder, 1991, Acta Cryst., vol B47, pp 617–630) or electron microscopy (see for example Santos et al., 2000, Materials Research, vol 3, No.4, pp 101–114). The structure of gamma-alumina is conventionally thought to approximate a spinel with a cubic form or a tetragonal form or combination.
In a common method of producing gamma-alumina, naturally occurring bauxite is transformed to gamma-alumina via intermediates. Bauxite is an ore, which is obtained from the earth's crust. Minerals commonly found in bauxite and the empirical formulas of their pure forms include gibbsite (α-Al2O3.3H2O), boehmite (α-Al2O3.H2O), diaspore (β-Al2O3.H2O), hematite (α-Fe2O3), goethite (α-FeOOH), magnetite (Fe3O4), siderite (FeCO3), ilmenite (FeTiO3), anatase (TiO2), rutile (TiO2), brookite (TiO2), hallyosite (Al2O3 2SiO2.3H2O), kaolinite (Al2O3 2SiO2.2H2O), and quartz (SiO2).
In a first transformation, gibbsite is derived from bauxite. The Bayer process is one common process for producing gibbsite from bauxite. The Bayer process was originally developed by Karl Joseph Bayer in 1888 and is the basis of most commercial processes for the production of gibbsite. As it is conventionally carried out, the Bayer process includes digestion of bauxite with sodium hydroxide in solution at elevated temperature and pressure to form sodium aluminate in solution, separation of insoluble impurities from the solution, and precipitation of gibbsite from the solution.
In a second transformation, boehmite is derived from gibbsite. As disclosed above, gibbsite is a trihydrated alumina having, in its pure form, the empirical formula α-Al2O3.3H2O. Transformation of gibbsite to boehmite may be accomplished by varying the conditions so as to influence the thermodynamic equilibrium to favor boehmite. For example, a method for producing boehmite from gibbsite may include dehydration in air at 180° C.
In a third transformation, gamma-alumina is derived from boehmite. Boehmite, in its pure form has the empirical formula α-Al2O3.H2O. Alternately, it is denoted in the art by γ-AlO(OH). The respective α and γ prefixes refer to the crystalline form. Boehmite is distinguished from other polymorphic forms of monohydrated alumina, such as diaspore (β-Al2O3.H2O), by its structure or crystalline form. In particular, boehmite typically has orthorhombic symmetry. Transformation of boehmite to gamma-alumina may be accomplished by varying the conditions so as to influence the thermodynamic equilibrium to favor gamma-alumina.
A support material is desirably stable. Under ambient (standard) conditions of temperature and pressure, such as for storage, gamma-alumina is less reactive and therefore more stable than boehmite. Thus, gamma-alumina is typically regarded as a more desirable support material than boehmite. Further, calcination of boehmite to form gamma-alumina before loading catalytic metal to the gamma-alumina is generally regarded as a desirable step in the formation of a catalyst from boehmite. Therefore, catalytic metal is typically not loaded to boehmite itself in forming a catalyst.
Despite the tendency of gamma-alumina to be stable at atmospheric conditions, gamma-alumina is known to exhibit a tendency to instability under hydrothermal conditions. For example, M. Abso-Haalabi, et al. in “Preparation of Catalysts V”, Ed. G. Poncelet, et al. (1991, Elsevier, Amsterdam, pp. 155–163) disclose that gamma-alumina undergoes an increase in average pore size and an accompanying decrease in surface area after hydrothermal treatment in the temperature range 150–300° C. Such a transformation would be undesirable in a catalyst. However, similar hydrothermal conditions occur, for example, in the Fischer-Tropsch process. In particular, in a Fischer-Tropsch process, water is produced during the Fischer-Tropsch reaction. The presence of water together with the elevated temperatures conventionally employed in the Fischer-Tropsch process create conditions in which hydrothermal stability, which is stability at elevated temperatures in the presence of water, is desirable. Fischer-Tropsch catalysts using gamma-alumina supports are known to exhibit a tendency to hydrothermal instability under Fischer-Tropsch operating conditions. This instability tends to cause a decrease in performance of gamma-alumina supported catalysts.
Consequently, there is a need for an improved support for a Fischer-Tropsch catalyst. Further needs include a Fischer-Tropsch catalyst that is hydrothermally stable under Fischer-Tropsch operating conditions. Additional needs include a catalyst that does not tend to decrease in performance under Fischer-Tropsch operating conditions.